Polymeric composites have been one of the most important materials during the last decades, they have the capacity to replace a lot of conventional materials i.e. metals. The use of natural fibre to make low cost and eco-friendly composite materials is a subject of great importance, particularly; wood fibres have recently attracted the interest of scientists and companies as reinforcement fillers due to its low cost, low density, high specific properties, less reliance on foreign oil sources, recyclability and carbon footprint reduction.
Some of the most common natural fibres are flax, hemp, coir, palm, cellulose, sisal, kenaf, bamboo, jute, wheat straw and wood fibres. However, some drawbacks of natural fibres (i.e. higher polar and hydrophilic) cause poorly compatibility between the natural fibres and the polymeric matrix. The synthetic polymers that are used as the matrix for natural fibres include thermoplastics and thermosets, however over the last decades; these matrices used are shifted from thermosets to thermoplastics.
The automotive industry have been instigated to develop new materials using these inexpensive fillers due to a new European Legislation implemented in 2006 which requires that in 2015 the 85% of the material of the car must be recycled or reused. The main benefit of these materials is the decrease of CO2 produced due to the origin of these materials and their low density and light weight. The weight reduction in the automotive sector decreases petrol consumption.
However, several limitations must be overcome in order to exploit the full potential of natural fibres. Firstly the fibre surface treatment needs to be well developed and implemented. Secondly properties of composites are greatly dependent on the content of fibre, matrix type and composite composition. The quality of fibre matrix interface should be improved for an optimized thermoplastic composition with perfect compatibilization between the natural fibres and the polymeric matrix.
Moreover, these natural-fibre reinforced plastics have some drawbacks, for example, the majority of the articles are used as off-screen structure pieces in interior parts due to its poor surface finish.
In order to improve the compatibility of natural fibres with synthetic polymer various modifications have been described e.g. alkali treatment, hot water treatment, silane treatment or salt water treatment. Those treatments may influence on the overall properties of the reinforced polymer including tensile strength and modulus, and flexural properties.
Aforementioned, natural-fibre reinforced plastics are used in numerous applications, i.e. automotive industry. In the past, the components (composites/plastics/articles/parts) were produced by compression moulding due to technical limitations in processing. Only recently has been possible to produce plastics materials in pellet form that could be processed by injection moulding technology where polypropylene (PP) is the most used matrix material.
In this context, thermoplastics natural fibre—synthetic resin composites based on polypropylene (PP) or polyethylene (PE) homopolymers have been developed in the last years for injection moulding applications due to ecological environment and light weight design, as disclosed in European patent application EP-A-2881249, German patent application DE-A-102012005127, and International patent applications WO-A-00/63285 and WO-A-2009/017387. However some drawbacks have been identified in those technical solutions, which can be associated to a complex injection process, low mechanical properties and poor surface finish to fulfil companies' requirements.
The use of natural fibre reinforced amorphous thermoplastic in particular acrylonitrile-butadiene-styrene (ABS) copolymers is limited.
Chinese patent application CN-A-102464850 relates to a natural fibre-reinforced styrene maleic anhydride copolymer compositions and deals with the poor compatibility of natural fibres. It is disclosed a general composition comprising 10-50 wt. % by weight of styrene-maleic anhydride copolymer, 10-40 wt. % of rubber elastomer, 5-25 wt. % of styrene-acrylonitrile copolymer, 5-40 wt. % of natural fibres, and 0.1-1.5 wt. % of processing aids selected from lubricants, antioxidants, mould release agents, or colouring agents, wherein the molecular weight of the styrene maleic anhydride copolymer is 20,000-300,000, the rubber elastomer is ABS (styrene-butadiene-acrylonitrile terpolymer), among others, and natural fibres are selected from sisal, ramie, flax, hemp, jute or bamboo.
Chinese patent application CN-A-103788566 relates to a plastic imitation of wood, which comprises the following components: 100 parts of ABS or polylactic acid (PLA), 5 to 70 parts of wood flour and 0.5 to 3 of a titanate coupling agent. The process for preparing it includes the treatment of wood flour with the coupling agent before the mixing with the thermoplastic polymer. This composition and process is suitable for preparing objects showing a wood-like texture with surface roughness, but not for smooth surfaces.
Japanese patent application JP-A-H09143378 refers to a thermoplastic resin composition, which comprises 10-98 wt. % of thermoplastic resin and 2-90 wt. % of wood flour having lignin content of 25 wt. % or below. The composition includes also a rubbery polymer, and it can include also a coupling agent and additives, such as a flame retardant, a lubricant, a plasticizer, antioxidants, antistatic agents, antimicrobial agents, foaming agents, silicone oils, and additives such as light resistance (weathering) agent.
Japanese patent application JP-A-2006/233111 refers to a styrenic resin composition, which can be easily extruded to obtain a grain pattern on the extruded object. In Example 1-5 it is disclosed a composition comprising 30 parts of ABS, 50 parts of styrene-acrylonitrile copolymer (SAN), 20 parts of SAN-butyl acrylate copolymer, 1 part of EBS, 1 part of calcium stearate, 1 part of magnesium stearate, 3 parts of PE wax, 1 part of oxidized PE wax, 15 parts of talc, 5 parts of wood flour, and titanium dioxide, among other pigments.
International patent application WO-A-2013/122649 discloses a composite polymer derived from melt processing of a thermoplastic polymeric matrix (45-85 wt. %) with wood pulp fibre (10-50 wt. %), preferably bleached chemical wood pulp fibre, which has a brightness of at least 20. It is disclosed that in the case of using nonpolar polymers, such as olefins, it is used a compatibilizer, typically graft copolymers such as maleic anhydride polypropylene or maleic anhydride polyethylene. To solve the problems associated with the uniform distribution of cellulose pulp fibres in the polymeric matrix, fibres are added to the polymer in a two-step operation process.
Chinese patent application CN-A-102924940 discloses an anti-aging plastic wood composite material prepared by the following raw materials by mass: 20%-35% of plastics, 20%-70% of natural wood fibers, 5%-45% of a fire retardant, 1%-10% of a compatibilizer, 0.1%-3% of a lubricant, 0.5%-3% of a plasticizer, 0.1%-2% of an anti-aging filling material, and 0.5%-3% of a coloring agent. It does not relate to the problem raised by the non-uniform distribution of natural wood fibers in the polymeric matrix.
Chinese patent application CN-A-104194125 discloses magnetic wood-plastic material, which comprises the following ingredients in parts by weight: 0.1-20 parts of rare earth magnetic powder, 0.5-5 parts of pigment, 10-70 parts of wood fibers, 20-85 parts of plastic, 2-5 parts of lubricant and 1-5 parts of compatilizer. It does not relate to the problem raised by the non-uniform distribution of natural wood fibers in the polymeric matrix.
Although different technologies have been developed to obtain articles with improved appearance, this target is not always reached using a single processing stage using natural-fibres reinforced thermoplastics compositions, let alone to produce a reinforced polymer capable of producing a coloured article with good surface appearance in a single stage.
There is, thus, a need to provide a thermoplastic composition, which overcomes the problems of the prior art technical solutions, being prepared by a simpler process, showing balanced mechanical and thermal properties and being suitable for extrusion, injection, and compression moulding as well 3D printing applications, including articles requiring excellent surface appearance.